This invention relates generally to the field of optical telecommunications, and more particularly to an instrument for measuring chromatic dispersion in optical fibers.
In general, all optical waveguides, and optical fibers in particular, exhibit dispersion. Dispersion generates a temporal broadening of optical pulses, which are the information carriers in optical telecommunications. This ultimately limits the transmission rate of the channel: the pulses overlap, and the ability to separate them is lost. In single-mode optical fibers the main factor of dispersion is chromatic dispersion: each frequency, or wavelength (i.e. color in the visible domain), propagates at a slightly different velocity. Since an optical pulse is built of a range of wavelengths, each of them propagating at a different velocity, chromatic dispersion is the main factor causing broadening of the pulses. Measuring chromatic dispersion in optical fibers, and compensating it with so-called chromatic dispersion compensators has become an essential issue.
In its recommendation ITU-T G.650, the International Telecommunication Union proposes three possible techniques for measuring the chromatic dispersion coefficient, which characterizes chromatic dispersion: the phase shift technique; the interferometric technique; and the pulse delay technique. For all these techniques, the chromatic dispersion coefficient is derived from a measurement of the relative group delay experienced by various wavelengths during propagation through a known length of fiber. The interferometric technique is designed for measurements of short lengths of fibers (having a length of meters). It is not adapted to long fibers (several kilometers) and installed cables, which is the domain addressed by the invention. Therefore, it will not be discussed further.
In the phase shift technique, which has been chosen as the reference technique, the group delay is measured in the frequency domain, by detecting, recording and processing the phase shift of a sinusoidal modulating signal. This method and some variations are used in all available commercial instruments, and are covered by several patents (references: U.S. Pat. No. 5,033,846 by Hernday et al., U.S. Pat. No. 5,406,368 by Horiuci et al.). This technique allows the most precise determination of the group delay (to the sub-picosecond), but requires delicate and expensive instruments.
The third method, based on a direct measurement of the group delay, has not yet found its way into commercial practice because it is difficult to combine in a satisfactory way the three main ingredients required: (1) fast optical pulses, either tunable or with a large spectrum; (2) fast detection, sensitive enough to detect the pulses; and (3) high resolution timing circuit.
Therefore, what is needed is a device and/or method of accurately measuring the chromatic dispersion coefficient over long distances which does not require delicate or expensive instruments.